Fiber-optic integrated membrane reactor

ABSTRACT

A reactor for water splitting or water treatment includes a first electrode, a second electrode electrically coupled to the first electrode, and a proton exchange membrane separating the first electrode and the second electrode. The first electrode includes a first optical fiber coated with a photocatalytic material.

This application is a divisional of U.S. patent application Ser. No.15/499,433 entitled “FIBER-OPTIC INTEGRATED MEMBRANE REACTOR” filed onApr. 27, 2017, which claims the benefit of U.S. Provisional PatentApplication 62/328,352 entitled “A FIBER-OPTIC INTEGRATED MEMBRANEREACTOR FOR PHOTOCATALYTIC WATER SPLITTING AND WATER TREATMENT” filed onApr. 27, 2016, both of which are incorporated by reference herein intheir entirety.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under CBET 1132779 andEEC-1449500 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

TECHNICAL FIELD

This invention relates to a reactor for photocatalytic water splittingand water treatment.

BACKGROUND

Photocatalytic water splitting is the photon-driven electrolysis ofwater to yield hydrogen (H₂) and oxygen (O₂). FIG. 1 depictsphotocatalytic electric cell 100, including anode 102, cathode 104,proton exchange membrane 106, and light source 108. Anode 102 isphotocatalytic. Light source 108 initiates oxidation at anode 102,yielding oxygen gas (O₂) and protons (H⁺). Reduction occurs at cathode104, where the protons are reduced to yield hydrogen gas (H₂). Hydrogengas can be used in industrial processing or as a mobile fuel source.However, development of commercial water splitting reactors has beenlimited by cost associated with process inefficiencies.

SUMMARY

Devices and methods for photocatalytic water splitting and watertreatment are described. In particular, an integrated reactor designincludes photocatalyst-coated optical fibers and photon exchangemembrane (PEM) elements incorporated into high surface area, high fluxmodules for production of hydrogen and purification of water. Thereactor design allows for use of natural and artificial light sourcestogether with immobilized photocatalysts tuned to absorb photons.Photocatalyst-coated optical fibers allow for illumination of a largesurface area of the photocatalyst via light transmitted through theoptical fibers. Compact water-splitting modules are formed by includinghollow-fiber PEM elements in various configurations. Decoupling of lightharvesting and irradiation from hydrogen generation via using fiberoptics to introduce light into a compact water-splitting module providesan efficient means to harvest gases (hydrogen, oxygen) that evolve fromthe catalyst.

In a first general aspect, a reactor for water splitting or watertreatment includes a first electrode, a second electrode electricallycoupled to the first electrode, and a proton exchange membraneseparating the first electrode and the second electrode. The firstelectrode includes a first optical fiber coated with a photocatalyticmaterial.

Implementations of the first general aspect may include one or more ofthe following features.

The first optical fiber may be coated with a conductive material. Thesecond electrode may include a second optical fiber, and the secondoptical fiber may be coated with a conductive material. The secondelectrode may be in the form of a flexible layer. The proton exchangemembrane may be hollow, and the first electrode may be positioned in theproton exchange membrane.

A light source may be coupled to the first optical fiber. The lightsource may be an artificial light source. The light source may be alight emitting diode. The first electrode may be configured to becoupled to a sunlight-collecting device or to a laser.

The proton exchange membrane may include a first layer and a secondlayer, and the first electrode may be positioned between the first layerand the second layer.

In a second general aspect, a reactor for water splitting or watertreatment includes a reservoir configured to hold water, a multiplicityof hollow fiber proton exchange membranes positioned in the reservoir, afirst electrode positioned in each of the hollow fiber proton exchangemembranes, where each first electrode includes a first optical fibercoated with a photocatalytic material, and one or more second electrodespositioned in the reservoir, each second electrode electrically coupledto at least one of the first electrodes.

Implementations of the second general aspect may include one or more ofthe following features.

The photocatalytic material may include titanium dioxide. The firstoptical fiber may be coated with a conductive material. The conductivematerial may include indium tin oxide.

A light source may be coupled to each of the first electrodes. The lightsource may be a light emitting diode.

In a third general aspect, a reactor includes a flexible assembly havinga first electrode layer, a second electrode layer, and a third electrodelayer. The first electrode layer includes a multiplicity of firstelectrodes positioned between a first proton exchange membrane layer anda second proton exchange membrane layer. Each of the first electrodesincludes an optical fiber coated with a photocatalytic material. Thesecond electrode layer includes a multiplicity of second electrodespositioned between a third proton exchange membrane layer and a fourthproton exchange membrane layer. Each of the second electrodes includesan optical fiber coated with a photocatalytic material. The thirdelectrode layer is positioned between and electrically coupled to thefirst electrode layer and the second electrode layer. The thirdelectrode layer is a flexible conductive material.

Implementations of the third general aspect may include one or more ofthe following features.

The flexible assembly may be wound around a porous conduit. The reactormay include a water inlet and a water outlet.

Thus, particular embodiments have been described. Variations,modifications, and enhancements of the described embodiments and otherembodiments can be made based on what is described and illustrated. Inaddition, one or more features of one or more embodiments may becombined. The details of one or more implementations and variousfeatures and aspects are set forth in the accompanying drawings, thedescription, and the claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a photocatalytic electric cell.

FIG. 2A depicts an exemplary photocatalytic reactor element. FIG. 2Bdepicts an exemplary photocatalytic reactor.

FIG. 3 depicts an exemplary photocatalytic reactor module including amultiplicity of photocatalytic reactor elements.

FIG. 4 depicts an exemplary spiral wound photocatalytic reactor.

FIG. 5 depicts simulation of biodegradation by growing phototrophs insoils through delivery of natural sunlight to the subsurface.

FIG. 6 depicts the sand packed optical fiber reactor (SPOFR) of Example1.

FIG. 7 shows dissolved oxygen levels at the inlet and outlet duringbioreactor operation described in Example 1.

FIG. 8 shows evanescent wave energy with respect to radial distance fromthe optical fiber surface as a function of photon incident angle forExample 2.

FIG. 9 shows light intensity absorbed in photocatalyst coated opticalfibers.

FIG. 10 shows light intensity absorbed in photocatalyst coated opticalfibers with fixed coating mass and increasing optical fiber dip coatinglength.

FIG. 11 shows degradation kinetics for methylene blue at different TiO₂doses and coating regimes.

FIG. 12 shows quantum efficiency, Φ, of methylene blue bleaching underphotolytic and photocatalytic conditions.

DETAILED DESCRIPTION

FIG. 2A depicts photocatalytic reactor element 200 including anode 202,cathode 204, proton exchange membrane (PEM) 206, and light source 208.Anode 202 is formed by a coating a portion of optical fiber 210 withconductive photocatalytic coating 212. Cathode 204 may be in the form ofan optical fiber with a conductive coating (e.g., platinum, carbonnanotube/graphene on carbon cloth or platinum mesh, or the like). PEM206 is a hollow fiber membrane. During operation, anode 202, cathode204, and PEM 206 are immersed in water. In some embodiments,photocatalytic reactor element 200 includes multiple anodes in PEM 206.For simplicity, FIG. 2A does not depict photocatalytic reactor element200 immersed in water. However, operation of photocatalytic reactorelement 200 is described with respect to FIG. 2A as if it were. Inembodiments that include multiple photocatalytic reactor elements 200,the number of anodes 202 may exceed the number of cathodes 204. That is,two or more photocatalytic reactor elements 200 may share a singlecathode.

Conductive photocatalytic coating 212 includes a photocatalyst as wellas a conductive material. An example of a suitable photocatalyst istitanium dioxide (TiO₂). An example of a suitable conductive material isindium tin oxide (ITO). Conductive photocatalytic coating 212 on opticalfiber 210 provides a large photocatalytic surface area for lighttransmitted from light source 208 through the optical fiber. Photonsabsorbed by the photocatalyst induce a current along the surface ofoptical fiber 210. The conductive material conveys the current alongoptical fiber 210 to cathode via circuit 214.

As depicted in FIG. 2A, water is oxidized at anode 202 to yield oxygengas and protons. PEM 206, a flexible, hollow fiber, separates anode 202from cathode 204 and allows protons to migrate from the anode to thecathode to achieve charge neutrality. In one example, PEM 206 has aninternal diameter of 2.2 mm (available from Permapure, USA). Protonsreact at cathode 204 to produce hydrogen gas. The hydrogen gas may becollected and stored as an energy source or for use in industrialchemical or water treatment processes.

Light source 208 may be a natural or artificial light source. Variouslight sources can be used to provide photons, and launched into thefiber optics that enter into the modules. For example, natural sunlightcan be concentrated using reflective surfaces. Light emitting diodes(LEDs) and lasers are examples of artificial light sources. Suitablelight sources can be polychromatic or monochromatic, based upon theabsorbance properties of the photocatalyst. Multiple light sources maybe used simultaneously, for different fibers, or to spandaytime-nighttime solar light availability.

The photocatalyst and the light source may be selected to maximizephoton absorption. As depicted in FIG. 2A, light source 208 is a lightemitting diode (LED) (e.g., 318 nm LED available from SETI, USA). Radiallight emission along the length of optical fiber 210 can be controlledby imparting a selected refractive index on the surface of the opticalfiber. In some examples, imparting a selected refractive index on thesurface of optical fiber 210 includes sheathing the optical fiber with amaterial such as glass, polymer, or the photocatalyst itself. Opticalfiber 210 provides a high surface area for reaction, and the closeproximity of the optical fiber to the PEM reduces the diffusion distancefor protons in the water, thereby increasing the overall rate andefficiency of reactor performance.

During operation of photocatalytic reactor element 200, light from lightsource 208 is provided to (launched into) optical fiber 210 ofphotocatalytic reactor element. Photons exit optical fiber 210 andexcite electrons in conductive photocatalytic coating 212 on the opticalfiber. The thickness and composition of the conductive photocatalyticcoating influences the wavelength of light absorbed, the extent ofelectron recombination, and the anodic potential. Electrons flow alongthe surface of the conductive photocatalytic coating 212 to cathode 204via conductor 214. Water can be split at anode 202 to produce oxygen andprotons. The protons diffuse through PEM 206 and produce hydrogen atcathode 204 using photogenerated electrons. Gases are collected at thetop of photocatalytic reactor element 200.

FIG. 2B is a cross-sectional view an embodiment of photocatalyticreactor 220 with anode 202, cathode 204, PEM 206, and light source 208.PEM 206 is a hollow fiber membrane. Optical fiber 210, coated withconductive photocatalytic coating 212, is coupled to anode 202. Photonsexit optical fiber 210 and excite electrons in conductive photocatalyticcoating 212 on the optical fiber. Electrons flow along the surface ofconductive photocatalytic coating 212 to cathode 204 via conductor 214.Anode 202 and cathode 204 are positioned in water 222 contained inhousing or reservoir 224. Oxygen gas exits photocatalytic reactor 220via outlet 226 proximate anode 202, and hydrogen gas exits thephotocatalytic reactor via outlet 228 proximate cathode 204.

FIG. 3 depicts exemplary photocatalytic reactor 300. Photocatalyticreactor 300 includes a multiplicity of photocatalytic reactor elements200. In some examples, photocatalytic reactor 300 includes tens orhundreds of photocatalytic reactor elements 200. Photocatalytic reactor300 may be implemented as a module in a larger photocatalytic reactorsystem. In some cases, the number of anodes in photocatalytic reactor300 is equal to the number of cathodes 204 in the photocatalyticreactor. In certain cases, the number of anodes in photocatalyticreactor 300 exceeds the number of cathodes 204 in the photocatalyticreactor, thereby reducing the use of expensive cathode materials.Cathodes 204 are typically positioned in close proximity to the anodes,thereby increasing reactor efficiency. Photocatalytic reactor elements200, including cathodes 204, are positioned in water in housing 302.Housing 302 may include reservoir 304 and cover 306.

During operation of photocatalytic reactor 300, water is provided tohousing via water inlet 308. The water may be provided to the hollowportion of PEMs 206. Water exits photocatalytic reactor 300 via wateroutlet 310. Light from light sources 208 is provided (launched) intooptical fibers of photocatalytic reactor elements 200. Photons exit theoptical fibers and excite electrons in the conductive photocatalyticcoating on the optical fibers. The thickness and composition of theconductive photocatalytic coating influences the wavelength of lightabsorbed, the extent of electron recombination, and the anodicpotential. Electrons flow along the surface of the conductivephotocatalytic coating to cathodes 204 via circuits (not shown). Watercan be split at the anodes (inside photocatalytic reactor elements 200)to produce oxygen and protons. The protons diffuse through PEMs 206 andproduce hydrogen at cathodes 204 using photogenerated electrons.Hydrogen is collected via conduit 312 at the top of photocatalyticreactor 300.

FIG. 4 depicts spiral wound photocatalytic reactor 400. Spiral woundphotocatalytic reactor 400 includes anodes 402 and cathode 404. Anodes402 may be similar to anodes 202 described with respect tophotocatalytic reactor element 200. Anodes 402 serve as spacers betweenPEM layers 406, and cathode 404 is positioned between two anode layersto yield a layered assembly in the form of a flat sheet. Cathode 404 isflexible, such that the layered assembly may be wound around porousconduit 408 to yield a compact module. Hydrogen produced along thecathode exits spiral wound photocatalytic reactor 400 through porousconduit 408. Water may be provided to PEM layers 406 at first end 410 ofspiral wound photocatalytic reactor 400 via an influent fitting, andwater and oxygen gas may exit the PEM layers at second end 412 of thespiral wound photocatalytic reactor via an effluent fitting. Duringoperation, spiral wound photocatalytic reactor 400 is positioned suchthat water entering the reactor flows downward through the PEM layers(e.g., via gravity), and hydrogen gas evolved at cathode 404 bubbles upand is collected at the top of the reactor. Water flow across the PEMlayers provides turbulence that improves evolution of hydrogen or othergases from the cathode.

Photocatalytic reactors described herein may be used to achieve waterpurification. For example, photo-generated electrons (and protons) maybe used to reduce oxidized pollutants (e.g., nitrate). Water flow may beincreased through the reactor, and a membrane may be selected tofunction as a PEM and a particulate/molecular cutoff filter to removeparticulate, colloidal, and/or dissolved ions. In some cases, thephotocatalyst may be selected to produce oxidants (e.g., hydroxylradicals), to oxidize organic pollutants and/or pathogens. Single ormultiple light sources may be used to deliver light of differentwavelengths into different fibers. Combinations of uncoated radialemitting fibers with UV light sources may be used for pathogeninactivation, while optical fibers coated with different catalystscreate different bandgaps appropriate for destroying different classesof inorganic and organic pollutants in water.

EXAMPLES Example 1: Remediation of Polluted Groundwater

Bacteria degrade organic groundwater pollutants (e.g.,trichloroethylene, methyl tert-butyl ether) when supplied with electrondonors (e.g., organic substances) and acceptors (e.g., oxygen). However,the inadequate supply and challenges in delivering oxygen to microbescreate an inactive environment with incomplete bioremediation.Biodegradation was simulated by growing phototrophs in soils throughdelivery of natural sunlight to the subsurface, as depicted in FIG. 5.With sufficient light, phototrophs convert carbon dioxide into cellularmaterials and generate oxygen. Oxygen acts as an electron acceptor toachieve contaminant biodegradation. Cellular and extracellular organicmaterial provide a substrate for a diverse group of heterotrophs to aidin pollutant degradation. Thus, carbon present in water and soils can becycled between inorganic and organic forms in a sustainable strategy,rather than continuous addition of electron donors (e.g., methanol).

Photosynthesis allows organisms to biologically produce oxygen. However,sunlight does not typically penetrate soil beyond ˜10 mm. Therefore, onefactor in using phototrophs in groundwater remediation is deliveringlight the treatment zone. This example demonstrates subsurfacephototroph growth using radially emitting optical fibers to direct lightto the subsurface. Oxygen was produced biologically to first enablemicro-organism growth and then degrade groundwater pollutants. A whitelight source was connected to a polymer covered optical fiber thatradially emitted light along its length, and the optical fiber wasinserted into a packed soil column. The polymer layer allowed the fiberto diffuse light radially along the soil column. Changes in dissolvedoxygen (DO) and microbial culture functional structure were monitoredwhen the light was provided continuously and in on-off cycles.

The ability to deliver oxygen and modulate redox conditions on diurnalcycles using solar light to remediate polluted groundwater wasdemonstrated. Visible light was delivered into the subsurface usinguncoated, radially emitting optical fibers. Phototrophic organisms grewnear the optical fiber in a packed sand column, and heterotrophicbacteria dominated at longer radial distances from the fiber, presumablysupported by soluble microbial products produced by the phototrophs.When applying light in on-off cycles, dissolved oxygen (DO) varied fromsuper saturation levels of >15 mg DO/L in the presence of light toundersaturated conditions of <5 mg DO/L in the absence of light. Theredox changes in response to light demonstrated biological delivery ofoxygen into the subsurface and supported a diverse microbial communityfor degrading organic or inorganic ground water pollutants.

Photobioreactor Design and Monitoring

A Sand Packed Optical Fiber Reactor (SPOFR), 70 cm long by 50.8 mmdiameter PVC single column, was packed with 61 cm playground sand, asdepicted in FIG. 6. A sponge scouring pad and a layer of granite wereplaced above and below the playground sand to prevent uneven water flow.Prior to the test, playground sand was mixed with photosyntheticorganisms from Tempe Town Lake, Arizona, grown in BG-11, a standardbiological growth medium. The composition of BG-11 is provided inTable 1. A 5 mm plastic optical fiber coated in white polymer to emitlight radially was positioned through the center of the column and wasconnected to an artificial visible light source (Encore Fiber OpticIlluminator model AR150CM-4-120M-D-CW with a 150W M type Lamp). Flowrate through the column was 100 mL/day, pumped from reactor bottom totop using a Cole Parmer Masterflex Peristaltic Pump (Vernon Hills,Ill.). An average light intensity of 20 μmol/s-m² in the visiblespectrum (400-700 nm) was emitted through the column continuously for 7days and then in a 24 hour on-light off-light cycle for 8 days torepresent a diurnal light cycle. DO exiting the sand column in responseto the light cycles was measured. A dissolved oxygen meter (YSI 550A,Yellow Springs, Ohio) measured influent DO by continuous stir of theinlet batch and effluent DO by inserting the meter in the air-tight flowpath. A 50 mL sample was collected for phosphate and nitrate analysisthrough a 50× dilution using a dual anion/cation ion chromatographyinstrument (ICS-5000, Dionex).

TABLE 1 BG-11 composition Compound mg/L Boric acid 2.86 Manganesechloride tetrahydrate 1.81 Zinc sulfate heptahydrate 0.22 Copper(II)sulfate pentahydrate 0.079 Colbalt(II) chloride hexahydrate 0.05 Sodiummolybdate 0.4 Dipotassium phosphate 30.5 Magnesium sulfate helptahydrate73.5 Calcium chloride 36.0 Ferric ammonium citrate 6 Sodium nitrate 750Sodium carbonate 20

Microbial Analysis

The density and biological diversity of bacteria along radial andlateral locations in the soil was quantified at the end of the test. TheSPOFR was cut into 8 cross sections of 76.2 mm in height. As depicted inFIG. 6, the six center sections were numbered 1-6 and used for microbialanalysis. Enumeration of phototrophic and non-phototrophicmicroorganisms was performed on sections 2, 4, and 6 to determine theradial and vertical distribution of the microorganisms. One gram of thehomogenously mixed cross-sectional area was sampled and diluted in 10 mLof water and serially plated onto BG-11 agar plates. The plates wereincubated with the same light intensity as the reactor at roomtemperature. Phototrophic and heterotrophic colonies were counted afterincubation for 5 days. DNA was extracted from sections 1, 3, and 5 from0.25 g samples using a MoBio PowerSoil® DNA Isolation Kit. Microbialcommunity amplicon sequencing using primers 515F and 806R for the V4hyper-variable region of the 16S rRNA gene was performed using theIllumina MiSeq platform at the Microbiome Analysis Laboratory in theSwette Center for Environmental Biotechnology at Arizona StateUniversity.

Oxygen Profiles

FIG. 7 shows the effectiveness of using fiber optics to increase oxygenin a subsurface environment. During days 1-6, continuous light wasprovided to the reactor filled with water at room temperature (22° C.).Oxygen saturation at 22° C. is 8.7 mg/L. The oxygen concentration of thereactor increased by 7 mg/L to a super-saturated level of 15 mg/L.During light on/off cycles, the DO fluctuated from over saturation tounder saturation in response to light delivery. Average DO levels forlight on and off cycles were 12.3 mg/L and 7.9 mg/L, respectively.

Increasing DO upon light application is indicative of aphotosynthetically active environment. Decreasing DO during off cyclesis indicative of consumption of oxygen for aerobic respiration, byphoto- or heterotrophic organisms. When no light was supplied to thereactor, DO decreased to 1.5-2 mg/L. This indicates that the oxygenincrease in the soil column corresponded to the light availability.During light application, photons promoted photosynthesis, whichincreased oxygen at a faster rate than the heterotrophs consume. Whenthere was no light, the heterotrophs consumed most of the oxygen.

High levels of nitrate and phosphate were used in the BG-11 media.Changes in these nutrients were additional indicators of biologicalactivity. Nitrogen decreased by an average of 50% from 526 mg/L to 260mg/L in the initial 5 days of testing then stabilized at a 10% decreaseof 70-80 mg/L. Phosphate decreased by 10.5 mg/L (day 4) to 11.7 mg/L(day 12) consistently throughout in both the light-on and light-offcycles. Both nitrogen and phosphorous are required for biomass growth,and the average consumption in these nutrients occurred at a ratio 18.6mg-N/mg-P, which is close to the redfield ratios in soil and soilbacteria. Nitrate can also act as an electron acceptor. As the oxygenlevel in the reactor increased, the nitrate consumption decreased.

A control reactor without an optical fiber or light resulted in a 79%decrease in DO (effluent column DO of 1.6 mg/L). Table 2 summarizes thedecrease in nitrate, phosphate, and oxygen during the test days. Theinlet nitrate and phosphate decreased by 1.5% and 15%, respectively,indicating that the majority of the nutrient reduction was microbiallyinduced and not accounted for by sand adsorption. Collectively, theSPOFR demonstrated production of DO in situ using phototrophic organismswith continuous flow through a simulated groundwater system.

TABLE 2 Reduction in nitrate, phosphate, and oxygen concentrations in acontrol with no light or photosynthetic organisms Concentrations ControlDay (mg/L) Inlet 1 2 3 4 Nitrate 526.7 446.7 450.2 445.6 448.3 Phosphate12.1 12.0 12.1 11.9 11.8 Oxygen 7.6 3.2 2.9 1.6 2.1

Biological Profiles

The microbial community profile, determined by plating and highthroughput sequencing, illustrated the success of the SPOFR insustaining a symbiotic community. Both phototrophs and heterotrophs werepresent. The average density of the heterotrophic microorganisms was1.19×10⁵±2.34×10; CFU/g of sand for all of the samples. The phototrophicmicroorganism density was consistent throughout the column at1.36×10⁵±2.28×10³ CFU/g of sand. Radial samples revealed 3.02×10⁵ CFU/gof phototrophs in the inner radius and a consistent 7.42×10⁴ CFU/gbeyond 7.62 mm radial distance from the fiber. Table 3 lists microbialdensity plating results.

TABLE 3 Microbial density plating results Microbial Density StandardStandard (CFU/g-sand) Phototrophs deviation Heterotrophs deviation 21.5E+05 6.5E+03 1.6E+05 5.3E+03 4 1.0E+05 6.9E+03 9.8E+04 2.6E+03 61.5E+05 9.1E+03 1.1E+05 6.0E+03 Effluent 0.0E+00 0.0E+00 9.5E+04 8.9E+034A 3.0E+05 3.8E+03 1.3E+05 1.9E+03 4B 7.4E+04 8.9E+03 1.4E+05 2.6E+03 4C7.5E+04 2.6E+03 9.6E+04 7.4E+03

The relative abundance of the microorganisms as determined by platingand high throughput sequencing illustrates that the majority of thephotosynthetic growth occurred within the first 7.62 mm of the radialprofile in both the spread plate and the DNA extraction. Thephotosynthetic organisms beyond 7.62 mm from the fiber are most likelydue to mixing from the water flow. There were no measured photosyntheticorganisms in the reactor effluent, suggesting that the photosyntheticmicroorganisms were tightly bound to the sand inside the bioreactor. Thelight delivered through the reactor dropped exponentially along thefiber length from inlet to outlet, from 20 μmol/s-m² at 0 m to 10μmol/s-m² at 1.52 m. The higher microorganism count in the top andbottom of the reactor is due to higher light exposure and highernutrient exposure, respectively.

An average of 60,000 taxonomy counts was found per sample in the SPOFR.Repeatability of the samples from the same area and the relativeabundance of each organism are listed in Table 4. The heterotrophs rangefrom nitrogen consuming (Nitrospirae) phylum to oxygen consuming phylaand are widely distributed in the environment. The phototrophs withinthe bioreactor were a mixture of algae (i.e., Acuodesmus obliquus andChlorotetraedron incuss) and cyanobacteria (Cyneccoccus spp.). Overall,the microbial community demonstrated a higher spatial distribution inthe inner radius of the column. This demonstrates that the opticalfibers successfully increased the oxygen to supersaturation levels (15mg/L) throughout a packed sand reactor. The oxygen increase in thereactor allowed for a syntrophic relationship and growth between thephototrophs and heterotrophs.

TABLE 4 Microbial distribution from DNA extraction in ten samplesOrganism 1_(a) 1_(b) 3A_(a) 3A_(b) 3B_(a) 3B_(b) 3C_(a) 3C_(b) 5_(a)5_(b) Cyanobacteria (all) 51.2% 54.4% 56.4% 62.9% 22.7% 20.4% 23.5%25.1% 36.9% 34.9% Cyanobacteria (other) 20.6% 21.9% 3.3% 3.7% 12.1%10.9% 13.4% 14.3% 5.6% 5.3% Cyanobacteria synecccoccus 19.2% 20.4% 45.2%50.4% 3.3% 3.0% 0.9% 1.0% 21.3% 20.2% Algae (Acutodesmus Obliquus) 6.6%7.0% 4.4% 4.9% 6.4% 5.7% 8.5% 9.1% 6.8% 6.5% Algae (ChlorotetraedronIncuss) 4.8% 5.1% 3.5% 3.9% 0.9% 0.8% 0.7% 0.8% 3.1% 3.0% Bacteroidetes20.8% 18.6% 16.9% 16.3% 25.0% 29.8% 33.2% 29.5% 26.6% 26.4%Proteobacteria 15.9% 15.3% 15.5% 12.0% 24.2% 22.4% 22.9% 19.9% 19.6%23.5% Nitrospirae 0.5% 1.0% 2.0% 1.6% 7.8% 6.5% 1.7% 5.8% 4.3% 4.9%Actinobacteria 1.1% 1.4% 2.5% 1.3% 5.0% 5.2% 4.3% 4.4% 3.7% 3.1%Verrucomicrobia 5.0% 3.5% 1.6% 1.5% 4.7% 6.8% 4.7% 5.3% 2.6% 3.0%Planctomycetes 0.7% 0.9% 1.6% 1.0% 2.7% 2.0% 3.2% 3.2% 1.6% 1.7%Armatimonadetes 1.5% 1.6% 1.1% 0.6% 4.4% 3.0% 2.5% 3.0% 2.3% 3.2%

Thus, radial emitting optical fibers have been used to deliver lightinto the subsurface and increase DO due to a biological response tolight stimulation. The nutrient consumption provided evidence forbiological reactions. Microbial community sequencing indicated that theorganisms present in the reactor included both heterotrophs, responsiblefor the consumption of nitrate and phosphate, as well as phototrophs,responsible for the production in oxygen through the column. There wasno reduction in flowrate across the column, suggesting soil clogging maynot be an issue.

Example 2: UV-LED Driven Photocatalyst Coated Optical Fiber System

A 318 nm ultraviolet light emitting diode (UV-LED) driven photocatalystcoated optical fiber (UV-LED-OF) system demonstrated in situ activationof photocatalysts by direct photon-electron transfer. Photocatalystimmobilized on coated optical fibers eliminated the need to recoversuspended photocatalysts from slurry suspensions, and photonic energylosses due to incident infrared heat dissipation, light transmittancethrough a liquid phase, or light scattering by suspended slurryphotocatalysts were minimized.

The capacity of a UV-LED-OF system with a 318 nm LED to transformmethylene blue (MB), a probe pollutant, was evaluated considering bothoptical fiber coating thickness and photocatalyst attachment method.Performance was compared with that of an equivalent-mass slurry catalystsystem. Predicted and measured photon fluence longitudinally through theoptical fibers decreased as a function of fiber length and mass of TiO₂externally coated on the fiber. Thinner coatings of TiO₂ on the opticalfibers led to faster removal rates of MB from solution, presumablyrelated with proximal distance between reactive species produced by TiO₂and MB in the solution. Dip-coated fibers with pre-synthesized TiO₂(i.e., electrostatic attachment) achieved faster MB removal than fiberscoated with TiO₂ using sol-gel and calcination techniques. TiO₂ attachedto optical fibers degraded MB faster and achieved a 5× higher quantumefficiency than an equivalent mass of TiO₂ suspended in a slurrysolution.

Photonic quantum efficiency was quantified as a function of opticalfiber coating thickness and photocatalyst synthesis and attachmentmethodologies. Electrostatic attachment on optical fibers of preformedTiO₂ was compared to sol-gel precipitation and calcination directly onoptical fibers. Experimental evidence utilizing methylene blue as aprobe compound was also supported by modeling.

Experimental Methods and Materials

Experiments with UV-LED-OF systems were conducted in a 10 mL glass batchreactor. This volume was selected to enable in situ quantification ofmethylene blue (Sigma Aldrich) in a spectrophotometer (HACH DR5000) at664 nm (95000 M⁻¹·cm⁻¹). This eliminated the need to remove samplevolumes from the reactor for analysis. A 318 nm UV-LED (SETi, UV-TOP)utilized 5 V input at <3 mA was mounted above the reactor. In mostcases, a single fiber was polished and attached to the LED. Opticalfibers, LED mounts, and additional polishing/stripping equipment werepurchased from Thorlabs: FT1000UMT; 0.39 NA, Ø1000 μm Core MultimodeOptical Fiber, High-OH for 300-1200 nm, TECS Clad. Optical fibers wereprepared by stripping the buffer and cladding, assembly into aquick-connect SMA connector, and polishing the optically activesurfaces. Polished fibers were then coated utilizing eitherelectrostatic dip-coating with preformed TiO₂ (P25 or P90 obtained fromEvonik) or sol-gel synthesis methodologies to achieve different layeringthickness and surface homogeneity. The mass of the TiO₂ coating layerson the optical fibers was measured gravimetrically by the weight of theoptical fibers before and after certain numbers of dip coating cycles(0-20 coating cycles). The surface morphology of the TiO₂ coating layerswas obtained by scanning electron microscopy (SEM/EDX: PhilipsXL30-EDAX). The thickness of the TiO₂ coating layers on the opticalfibers was obtained from SEM images of vertically oriented fibers.

Fiber Stripping and Preparation for Catalyst Deposition

11 cm fiber segments were cut utilizing a ceramic square or ruby bladeto achieve a clear cut of the fiber. To remove the TECS cladding andbuffer underneath, the fibers were soaked in acetone for 24 hours andthe cladding was manually removed. As needed, after a further 24 hoursof soaking in acetone, the remaining cladding (a clear coating aroundthe fibers that maintains total internal reflection of the light) wasstripped manually. The fibers were then left to soak in water to removeany remaining acetone residual as the catalyst solution was prepared.

Fiber Mounting and Polishing for Enhanced Light Transmission

Fibers were fixed to the metal connector (SMO5SMA, Thor Labs) utilizingheat shrink wrap (TT100 1/16″ and ⅛″, 0.5 cm and 1.5 cm, respectively,Tech-Tron) placed (1) between the stripped fiber and the connector and(2) overlaid on the combined fiber-connector. Heated air was utilized toshrink wrap the components to flush-fit and allowed to cool prior tofurther treatment or use. Thor Labs ruby blade was utilized to gentlyscore the fiber and cleave along the mechanical axis of the fiber toachieve smooth ends for polishing. Mounted, cleaved fibers were thenpolished utilizing (D50SMA, Thor Labs) polishing assembly with fiberpolishing paper (LF30P, LF5P, LF03P). Fiber microscope was used todetermine uniformity of clarity at the fiber tip.

LED mounting to the optical fiber employed a butt-coupling method ofdirect contact between the LED quartz window and polished optical fibertip (S5LEDM, SM05M05, SM05SMA, Thor Labs). Male/female SMA pairingallowed for LED housing to connect directly to polished fiber assembly.

Dip Coating Method for TiO₂ Deposition

A dispersion of 1% or 2% (10 g/L, 20 g/L) TiO₂ (P90, Evonik) was createdutilizing the CEINT protocol. The dispersion solution was nanopure waterand the appropriate loading of P90 was added to reach 20 g/L, with noadditional chemical addition (pH=4.0-4.5). The solution was sonicated inan immersed sonicator horn and maintained stability for up to 48 hours.Two variations of a dip coating protocol were utilized: (1) extendeddip/dry cycling and (2) rapid-enhanced dip/dry cycling. For (1), fiberswere immersed in the solution for 24 hours to achieve good electrostaticinteraction; fibers were then allowed to dry (24 hours) and rinsed withnanopure water to release any excess TiO₂ prior to analysis or use. In(2), 30 s dip/dry cycling was conducted with a 2% TiO₂ solution for TiO₂deposition and hot-air drying to allow for rapid processing of theoptical fibers.

Sol-Gel Method for TiO₂ Deposition

To functionalize the optical fiber surface, the optical fibers werefirst sonicated for 30 min in acetone, ethanol and water, respectively,then rinsed with distilled water and dried with a stream of nitrogengas. Then the optical fibers were immersed in piranha solution (H₂O₂:H₂SO₄=1:3, volume ratio) to generate a hydroxyl-functionalized surface.The functionalized optical fibers were rinsed with water and ethanol,respectively, then immersed into 6 mL of ethanol (200 proof) containingtitanium isopropoxide (TTIP, 0.6 ml). After 5 min, the fibers wereslowly taken out and exposed to air for another 5 min to allow thehydrolysis of TTIP to generate a layer of TiO₂ precursor. Repeating thedip-coating process resulted in multiple layers of TiO₂ precursor.Optical fibers were dried at 60° C. for 2 h to allow completehydrolysis, then heated to 500° C. (2° C./min) for 1 h to crystallizethe TiO₂ particles. As a control, P25 (1 wt %) instead of TTIP wasdispersed into ethanol, and used as precursor solution for dip-coating.The samples were characterized by SEM.

Optical fibers were immersed into the reactor containing a test solutionwith 4.0 μM methylene blue, as a probe contaminant, in double deionizedwater at a resistivity above 18.2 MΩ-cm (Millipore Inc.). Directphotolysis experiments were conducted by utilizing a 1 cm uncoatedoptical fiber to launch 318 nm LED irradiation into the solution. Lightintensities emitted from the LED and terminal end of the fiber tips weremeasured by a radiometer (Avaspec 2048L). MB concentrations weremeasured by absorbance at 664 nm.

Theoretical Calculations of Light Interactions with Coatings on OpticalFibers

Assessment of the mechanism for light penetration from the optical fiberinto the catalyst and surrounding aqueous solution helped determinepathways of photon delivery and anticipated photon-electron conversionyields. With coated photocatalyst thickness and mass as primaryvariables, mathematical modeling of the photonic penetration depthinformed desired experimental coating thicknesses.

Excitation in the fiber photocatalyst system is attributed to evanescentenergy emitted from optical fibers. Light reflects along the fiber dueto total internal reflection (TIR) provided the input angle is greaterthan the critical angle (ec):

$\begin{matrix}{\theta_{c} = {\sin^{- 1}\left( \frac{n_{2}}{n_{1}} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where n₁ is the index of refraction inside the fiber and n₂ is the indexoutside the fiber (derived from Snell's law with the refracted angle setto 90°, requires n₂<n₁). During TIR, the boundary conditions ofMaxwell's equations result in an imaginary wavenumber (k), whichproduces a corresponding energy field:

k=k _(y) ŷ+k _(x) {circumflex over (x)}=iaŷ+β{circumflex over(x)}  Equation 2

E({circumflex over (r)})=E ₀ e ^(−i(iaŷ+β{circumflex over (x)})) =E ₀ e^(+ŷ−iβ{circumflex over (x)})  Equation 3

The flow of this energy (E), called an evanescent wave, is parallel tothe waveguide surface, while the z component of the wave (perpendicularto the waveguide) falls off exponentially so that only a limited amountof energy is transmitted into the second medium.

$\begin{matrix}{{{E(z)} = {{{E(0)}e^{- \frac{z}{d}}} = {\frac{hc}{\lambda}e^{- \frac{z}{d}}}}};{d = \frac{\lambda}{4\pi \sqrt[2]{{n_{1}^{2}\sin^{2}\theta} - n_{2}^{2}}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

This rapidly decaying energy field is dependent on the wavelength of thelight (λ), the incident angle inside the waveguide (θ), and both n₁ andn₂. FIG. 8 shows plots of this function in the present system (λ=318 nm)for incident angles 75°, 80°, 85°, and 90° (θ>74.5° due to the numericalaperture of 0.39). The evanescent field energy falls off as a functionof distance from the waveguide surface. The solid horizontal line on thegraph shows the bandgap energy necessary (3.2 eV) to excite the TiO₂photocatalyst. The evanescent wave has enough energy to create aphotocatalytic exciton in the first ˜10 nm outside the fiber (i.e.,thickness of photocatalyst coating), a distance comparable to theprimary particle size of the TiO₂. In this region, the energy is abovethat of the band gap for TiO₂. This modeling suggests that thinnercoatings may be more efficient in activating all the TiO₂ on thesurface.

TiO₂ Coating Characterization

Based on the theoretical mechanism for optical fiber light-energytransmission, the dependence of photocatalytic performance on coatedthickness, length of optical fiber, and applied light intensity wasinvestigated. The dip coating method yielded 0.02 mg TiO₂ on every 7 cmoptical fiber for each coating cycle; up to 20 coating cycles wereperformed. The thickness of the TiO₂ coating layers on the opticalfibers increased from 0 nm (uncoated) to 1150±60 nm up to 5 coatingcycles, then stayed at about 1150 nm even with more coating cycles. Themass increase on the fiber after five coating cycles was attributed tothe filling up of cavities on the TiO₂ coating layers. SEM of sol-gelsamples from titanium isopropoxide (TTIP) and P25 precursors confirmmore holistic coverage at for five coating cycles compared to onecoating cycle, but do not have the compaction observed with water-baseddip-coating.

Optimizing Catalyst-Absorbed Light Intensity

For several different 28 cm long optical fibers coated with between 0.02mg and 0.1 mg of electrostatically attached TiO₂, FIG. 9 shows thathigher coated masses of TiO₂ resulted in an increase in light intensityabsorbed by TiO₂ from 1.71×10⁻¹² (±5.23×10⁻¹³) Einstein·cm⁻²·s⁻¹ to5.36×10⁻¹² (±3.33×10⁻¹³) Einstein·cm⁻²·s⁻¹. Light may generate HO.,generate heat (i.e., recombination of electrons and holes), or becomescattered. The light intensity absorbed by TiO₂ and the coatingthickness followed an attenuating trend, where additional coating cyclesdid not correspond to increased light flux into the catalyst.

Another set of experiments was conducted with variable optical fiberlengths but a fixed deposited phototcatalyst mass (0.02 mg TiO₂) andresulting fixed thickness of TiO₂. FIG. 10 shows the light intensityabsorbed by TiO₂ increased from 7.20×10⁻¹³ (±1.39×10⁻¹))Einstein·cm⁻²·s⁻¹ to 2.15×10⁻¹² (±2.99×10⁻¹³) Einstein·cm⁻²·s⁻¹ atincreasing length of the coated fiber from 7 cm to 28 cm. The lightintensity absorbed by TiO₂ exhibited a linear relationship(I=(8×10⁻¹⁴)×Length, R²>0.98) as a function of the optical fiber length(cm). At an added mass of 0.4 mg, the light intensity absorbed by theTiO₂ increased more readily with coated length, from 2.85×10⁻¹²(±7.90×10⁻¹⁴)) Einstein·cm⁻²·s⁻¹ to 5.51×10⁻¹² (±1.14×10⁻¹³)Einstein·cm⁻²·s⁻¹ at 7 cm and 28 cm, respectively.

Quantum Efficiency of the AM-LED-OF System

FIG. 11 shows the transformation kinetics of MB at different UV doses.The degradation of MB followed the pseudo-first order kinetics. WithoutTiO₂ present in slurry form or attached to the optical fiber (control),less than 5% of the MB degraded over the 4 hour test. Faster MBdegradation occurred when TiO₂ was present. For TiO₂ present in themixed slurry reactor, MB degradation rates increased with higher TiO₂dosages but plateaued for dosages above 5 mg/L. It is thought that theremay be a limited reaction zone where light penetration into thewater-TiO₂ slurry occurs and leads to MB degradation. MB degradationrates in the TiO₂ slurry reactor were always slower in experiments withequal TiO₂ mass attached to optical fibers and the same lightirradiance. This suggests that the MB degradation by the slurry systemwas less effective than that by the fibers.

FIG. 11 also shows that the degradation rates of MB by the dip-coatedTiO₂ optical fibers at the added mass of 0.2 mg and 1 mg were similar(1%). Although more light was absorbed as the coating layer becamethicker as shown in FIG. 9, the extra light absorbed by the thickercoating layer did not produce additional reactive performance. Thethicker coating layer enhanced the light absorption, but it may havecreated barriers for MB to transfer to inner reactive sites on the TiO₂coating layer, creating a non-reactive inert TiO₂ coating zone.

An inverse relationship between coating thickness and performance wasobserved in the sol-gel coated fibers. The kinetic experimental datashown in FIG. 11 is consistent with the theoretical results shown inFIG. 8, indicating that increasing the thickness of the TiO₂ coating didnot increase the photocatalytic degradation rate.

For comparison, the quantum efficiency (QE) of the dip-coated andsol-gel coated fibers and slurry systems at equivalent catalyst doseswere calculated as follows:

${{QE} = \frac{- {k\lbrack{MB}\rbrack}}{I_{{ab}\; s}}},$

where k is the pseudo first order reaction rate of MB degradation indifferent systems, [MB] is the initial methylene blue concentration, andI_(abs) is the light intensity absorbed by the TiO₂ coating layer. Dueto the direct absorption of light by MB at 318 nm, photolysiscontributes additively to the bleaching of MB. FIG. 12 shows QE forphotolysis, various slurry conditions with P25 and P90, and dip-coatedor sol-gel coated 7 cm optical fiber with one or five coating cycles(0.2 mg to 1 mg equivalent catalyst mass added). The initial MBconcentration was 4 μM, and the reactor volume was 10 mL. Slurry basedsystems are seen to have marginally higher quantum efficiency (QE) thanphotolysis alone. In contrast, QE for the coated optical fibers was 3×to 10× greater than photolysis alone. The systems with the highest QEwere single dip-coating or single sol-gel coating. The data indicates ahigher energy utilization efficiency of the fibers compared to theslurry system. Thus, the UV-LED/TiO₂/optical-fiber system provides amore energy efficient way to remove pollutants when directly compared toa slurry system of equivalent inputs.

Direct coupling of LEDs to photocatalyst coated optical fibers for insitu irradiation of fixed-film photocatalysts has been demonstrated toremove pollutants in water. Enhanced performance of coated fiberscompared to equivalent slurry conditions indicates photon-electron/holeconversion yielding oxidation reaction of the methylene blue in acontrolled-catalyst delivery configuration. LEDs provide anarrow-wavelength output for irradiation, capable of targetingpollutants via photolysis or photocatalytic mechanisms while decreasingrequired energy inputs and systemic inefficiency due to heat losses.

Further modifications and alternative embodiments of various aspectswill be apparent to those skilled in the art in view of thisdescription. Accordingly, this description is to be construed asillustrative only. It is to be understood that the forms shown anddescribed herein are to be taken as examples of embodiments. Elementsand materials may be substituted for those illustrated and describedherein, parts and processes may be reversed, and certain features may beutilized independently, all as would be apparent to one skilled in theart after having the benefit of this description. Changes may be made inthe elements described herein without departing from the spirit andscope as described in the following claims.

1-17. (canceled)
 18. A reactor comprising: a flexible assemblycomprising: a first electrode layer comprising a multiplicity of firstelectrodes positioned between a first proton exchange membrane layer anda second proton exchange membrane layer, wherein each of the firstelectrodes comprises an optical fiber coated with an electricallyconductive, photocatalytic material; a second electrode layer comprisinga multiplicity of second electrodes positioned between a third protonexchange membrane layer and a fourth proton exchange membrane layer,wherein each of the second electrodes comprises an optical fiber coatedwith an electrically conductive, photocatalytic material; and a thirdelectrode layer positioned between and electrically coupled to the firstelectrode layer and the second electrode layer, wherein the thirdelectrode layer is a flexible electrically conductive material. 19.(canceled)
 20. (canceled)
 21. The reactor of claim 18, wherein theflexible assembly is in the form of a sheet.
 22. The reactor of claim21, wherein the flexible assembly is wound around a porous conduit. 23.The reactor of claim 22, wherein the porous conduit has a first end anda second end.
 24. The reactor of claim 23, further comprising a waterinlet, wherein the water inlet is configured to direct water toward thefirst end of the porous conduit.
 25. The reactor of claim 24, furthercomprising a water outlet, wherein the reactor is configured to allowwater to flow from the first end of the porous conduit toward the secondend of the conduit via gravity.
 26. The reactor of claim 18, furthercomprising a light source coupled to each of the first electrodes andeach of the second electrodes.
 27. The reactor of claim 26, wherein thelight source is an artificial light source.
 28. The reactor of claim 27,wherein the light source is a light emitting diode.
 29. The reactor ofclaim 18, wherein each of the first electrodes and second electrodes isconfigured to be coupled to a sunlight-collecting device.
 30. Thereactor of claim 18, wherein each of the first electrodes and secondelectrodes is configured to be coupled to a laser.
 31. The reactor ofclaim 18, wherein each of the first electrodes and second electrodes iscoated with an electrically conductive material.
 32. A method ofproducing hydrogen and oxygen from water, the method comprising:providing water to a reactor comprising: a reservoir; and a flexibleassembly positioned in the reservoir, the flexible assembly comprising:a first electrode layer comprising a multiplicity of first electrodespositioned between a first proton exchange membrane layer and a secondproton exchange membrane layer, wherein each of the first electrodescomprises an optical fiber coated with an electrically conductive,photocatalytic material; a second electrode layer comprising amultiplicity of second electrodes positioned between a third protonexchange membrane layer and a fourth proton exchange membrane layer,wherein each of the second electrodes comprises an optical fiber coatedwith an electrically conductive, photocatalytic material; and a thirdelectrode layer positioned between and electrically coupled to the firstelectrode layer and the second electrode layer, wherein the thirdelectrode layer is a flexible electrically conductive material, whereinthe flexible assembly is wound around a porous conduit; providing lightinto the optical fibers of the first and second electrode layers,thereby yielding photogenerated electrons, wherein the photogeneratedelectrons flow along the electrically conductive, photocatalyticmaterial of the first and second electrode layers to the third electrodelayer; and converting the water into oxygen gas and hydrogen gas. 33.The method of claim 32, wherein the water flows through the protonexchange membrane layers from a first end of the reactor to a second endof the reactor via gravity.
 34. The method of claim 33, wherein theoxygen gas exits the reactor at the second end of the reactor.
 35. Themethod of claim 34, wherein some of the water exits the reactor at thesecond end of the reactor.
 36. The method of claim 33, wherein thehydrogen gas exits the reactor at the first end of the reactor.
 37. Themethod of claim 36, wherein the hydrogen gas exits the reactor throughthe porous conduit.
 38. The method of claim 32, wherein a flow of thewater across the proton exchange membrane layers provides turbulence,thereby improving evolution of the hydrogen gas.
 39. The method of claim32, wherein converting the water into oxygen gas and hydrogen gascomprises producing hydrogen at the third electrode layer using thephotogenerated electrons.